Jason Collins, a veteran centre in the National Basketball Association (NBA), announced on Monday that he was gay, breaking one of the final frontiers in US sports and society.

Collins became the first active player from any of the four major US men's professional sports leagues to publicly reveal his homosexuality.

He did so in a first-person account published in Sports Illustrated, saying he had gradually become frustrated with having to keep silent on gay issues. The Boston Marathon bombings this month had convinced him not to wait any more for a perfect moment to come out, he wrote.

"I'm a 34-year-old NBA centre. I'm black. And I'm gay. I didn't set out to be the first openly gay athlete playing in a major American team sport. But since I am, I'm happy to start the conversation," Collins said.

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"I wish I wasn't the kid in the classroom raising his hand and saying, 'I'm different.' said Collins, who played last season with the Boston Celtics and then the Washington Wizards and is currently a free agent.

Players, administrators and some politicians applauded him for taking a stance. Some hailed it as a landmark day in American civil rights, as important as when Jackie Robinson broke the colour barrier in baseball.

Collins' move came at a time of heated debate over gay rights in the United States, where polls show public opinion is fast moving toward greater acceptance, although a core of social conservatives oppose such change.

In the coming months, the Supreme Court will rule on whether to strike down parts of a federal law that defines marriage as the union between a man and a woman. In 2011, the military repealed a ban on openly gay soldiers.

"Jason's announcement today is an important moment for professional sports and in the history of the LGBT (lesbian, gay, bisexual, transgender) community," former US president Bill Clinton said in a statement.

NBA commissioner David Stern said he was proud of Collins.

If I had my way, someone else would have already done this. Nobody has, which is why I'm raising my hand.

"Jason has been a widely respected player and teammate throughout his career and we are proud he has assumed the leadership mantle on this very important issue," Stern said.

In a country where it is no longer news for politicians and entertainers to be openly gay, the absence of an openly gay male player in any of the major professional sports had become a hot topic.

Sports, which helped play a key role in changing public opinion on racial discrimination, had come to seem out of step with much of the rest of American society.

Collins, who is 34, and who has played with six different teams during his 12 years in the NBA, said he never had any grand plans of being the first openly gay player, but events off the basketball court persuaded him to come out.

He was inspired by last year's gay pride parade in Boston, he said, but delayed making an announcement due to a desire to protect his team, waiting until the end of the regular 2012-2013 season ended. Collins was also prompted by the April 15 Boston Marathon bombings which killed three people and wounded more than 200, he said.

"The recent Boston Marathon bombing reinforced the notion that I shouldn't wait for the circumstances of my coming out to be perfect," he wrote in Sports Illustrated. "Things can change in an instant, so why not live truthfully?

Kobe Bryant, one of the NBA's greatest players, was among dozens of active players who took to social media to applaud Collins.

"Proud of @jasoncollins34. Don't suffocate who u r because of the ignorance of others," Bryant tweeted.

Two-time NBA Most Valuable Player Steve Nash tweeted: "The time has come. Maximum respect."

A-list celebrities also were quick to back Collins -- from movie director Spike Lee, a front-row spectator at New York Knicks games, to US talk show host Ellen DeGeneres, who herself made headlines when she came out.

There are openly gay players in many top professional leagues in other countries in the world as well as smaller leagues in North America and individual sports.

But there has been no active player from the big four pro men's leagues - the NBA, the National Football League, the National Hockey League and Major League Baseball - who had come out until now.

Bill Clinton, whose daughter Chelsea was a classmate of Collins at Stanford University, said he hoped Collins would be treated fairly by everyone.

"I hope that everyone, particularly Jason's colleagues in the NBA, the media and his many fans extend to him their support and the respect he has earned."

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A sense that it was hard for gay athletes to come out had started to change in recent years, and it had seemed like only matter of time until an active male player in one of the big pro leagues said he was gay.

The question came into sharp focus this year around the National Football League (NFL), usually viewed as the most macho of America's pro sports.

In the days leading up to this year's Super Bowl in New Orleans in February, San Francisco 49ers cornerback Chris Culliver told reporters he would not welcome a gay teammate into the locker room.

He later retracted his comments but reports later emerged of NFL teams asking college players about their sexuality at a scouting session, or combine, in February.

This prompted the New York State attorney general to send a letter to the NFL, urging the league to take action and adopt a formal policy on sexual discrimination.

High-profile NFL players, most notably Chris Kluwe and Brendon Ayanbadejo, began advocating for gay rights, and suggested there were a handful of players ready to come out once someone had taken the first step.